1. Technical Field
The present invention relates to an electrostatic transducer. In particular, the invention relates to an electrostatic transducer having a driving circuit, which is suitable for an electrostatic transducer that reproduces a sound having high directionality by outputting a modulated wave obtained by modulating a carrier wave in an ultrasonic band with a sound signal in an audible band, to a method of setting a circuit constant of the electrostatic transducer, to an ultrasonic speaker, to a display device having the ultrasonic speaker, to a directional sound system, and to a driving circuit of a capacitive load.
2. Related Art
An ultrasonic speaker can reproduce a sound having high directionality by outputting a modulated wave obtained by modulating a carrier wave in an ultrasonic band with a sound signal in an audible band. In general, a piezoelectric transducer is used as a transducer (transmitter) of the ultrasonic speaker. However, since the piezoelectric transducer uses a sharp resonance characteristic of an element, a frequency band thereof is very narrow even though high sound pressure can be obtained. As a result, in an ultrasonic speaker using the piezoelectric transducer, a reproducible frequency band is narrow, and accordingly, the reproduced sound quality tends to be poor as compared with a loudspeaker. For this reason, a variety of studies for improving the reproduced sound quality is being made (for example, refer to JP-A-2001-86587).
In addition, there is an ultrasonic speaker using an electrostatic transducer (refer to an example of an electrostatic transducer shown in FIGS. 6A to 6C) in which an electrostatic force is applied between an electrode of a vibrating film and a fixed electrode so as to vibrate the vibrating film and generate the sound pressure. The electrostatic transducer is characterized in that a flat output sound pressure characteristic can be obtained over a wide frequency range. Accordingly, in the ultrasonic speaker using an electrostatic transducer, the reproduced sound quality can be improved, as compared with the ultrasonic speaker using a piezoelectric transducer.
However, in the case of driving the electrostatic transducer in an analog amplifier, the following problems occur.
FIGS. 12A and 12B are views illustrating examples of a single end push-pull circuit. Referring to FIGS. 12A and 12B, it will be described about a difference between a case of driving a resistive load and a case of driving a capacitive load in a typical analog power amplifier. As shown in FIGS. 12A and 12B, the typical analog power amplifier uses a single end push-pull circuit in which an NPN transistor Tr1 and a PNP transistor Tr2 are totem-pole-connected to each other at an output stage (power amplification stage) in an up and down direction. An output-stage transistor is configured to operate in a class AB (class B) or a class A. In addition, FIG. 12A illustrates an example of the case of driving a load resistor RL serving as a resistive load, and FIG. 12B illustrates an example of the case of driving a load capacitor CL serving as a capacitive load.
FIGS. 13A and 13B are views illustrating examples of a power loss occurring in an output-stage transistor (one side) of an analog power amplifier. Specifically, FIGS. 13A and 13B illustrate the relationship between a collector current IC and a voltage VCE between a collector and an emitter of the upper transistor Tr1 shown in FIGS. 12A and 12B in the case when an output-stage transistor operates in a class B. In the case of the resistive load, the phase of an output voltage (load voltage) and the phase of an output current (load current) are approximately equal to each other, and accordingly, the collector current IC and the voltage VCE between the collector and the emitter of the transistor have an inverted relationship as shown in FIG. 13A. That is, the voltage VCE is a minimum when the output current IC is a maximum, and the voltage VCE is a maximum when the output current IC is a minimum.
In contrast, in the case of the load capacitor CL, the phase of the output voltage (load voltage) and the phase of the output current (load current) are different from each other by 90°, and accordingly, the phase of the voltage VCE and the phase of the current IC are also different from each other by 90° as shown in FIG. 13B. At this time, when the output current IC is a maximum, the voltage VCE is not a minimum but is high, and thus a large loss WQ occurs in the transistor. As a result, a power loss larger than in the case of the resistive load occurs in the transistor.
As described above, when an electrostatic transducer is driven by the typical analog power amplifier, the power loss in the output-stage transistor is larger in the case of the capacitive load than the case of the resistive load assuming that output power is equal. Consequently, in the case when the electrostatic transducer is driven by the analog power amplifier, a power amplifier having an output higher than in the case of driving the resistive load is required, which causes a problem in that an apparatus becomes large.
On the other hand, in recent years, a class-D power amplifier that causes an output-stage transistor to switching-operate has come into wide use as an audio power amplifier (for example, refer to JP-A-2002-158550). The class-D power amplifier is characterized in that a power MOSFET having low on-resistance is used as an output-stage element and it is possible to reduce a loss in the output-stage element by performing a switching operation on the MOSFET. Since the loss in the output-stage element is small in the class-D power amplifier as compared with an analog amplifier, a radiator indispensable to the analog power amplifier may not be prepared or may be made small.
Therefore, it is possible to realize a small and high-output amplifier. For this reason, the class-D power amplifier is often adopted in an amplifier for a vehicle or an amplifier for a portable terminal, in which miniaturization and low loss is required, an AV amplifier having a large number of output channels, or the like.
FIG. 14 is a view illustrating an example of the configuration of a typical class-D power amplifier. In the class-D power amplifier 21 shown in FIG. 14, a PWM circuit 41 modulates an input signal 40 to a high-frequency digital signal by using a PWM (pulse width modulation) method or a PDM (pulse density modulation) method and then a gate driving circuit 42 drives a class-D output stage 43. In the class-D output stage 43, the power MOSFET having low on-resistance is used such that the power MOSFET operates in a saturated region, that is, performs a switching operation (ON/OFF operation), by means of the gate driving circuit 42. While the power MOSFET is in an OFF state, a current hardly flows, and accordingly, the loss in the power MOSFET is almost zero. On the other hand, while the power MOSFET is in an ON state, a current flows through a load; however, since a resistance of the power MOSFET being in the ON state, that is, a so-called ON resistance is so small as to be within a range of several to several tens of milliohms, the loss in the power MOSFET can be suppressed to be significantly small even if a large amount of current flows. Accordingly, since the loss occurring in an output-stage element is very small in the class-D power amplifier 21 as compared with an analog amplifier, it is possible to realize a small and high-output amplifier.
As described above, the output of the class-D output stage 43 is a switching wave (modulated wave), the output of the class-D output stage 43 needs to be supplied to a load after eliminating switching carrier components by the use of a low pass filter. As the low pass filter, an LC filter in which the power loss is small is generally used.
Here, a case in which a capacitive load such as an electrostatic transducer is driven by the class-D power amplifier will be considered. As described above, in order to eliminate the switching carrier components, an LC filter is inserted behind a class-D output stage in the class-D power amplifier. Here, a capacitor C, which is a part of the LC filter, may be replaced by an electrostatic transducer. That is, the load capacitor C may be used as a part of the LC filter.
FIG. 15 is a view illustrating an example of the configuration of a class-D power amplifier that uses a fourth-order LC low pass filter. In the case of a typical audio power amplifier, a load to be driven, which is shown in FIG. 15, is a resistive component (load resistor RL). On the other hand, in the case when an electrostatic transducer is to be driven, it may be considered that a capacitor C2, which is a part of the LC filter, is replaced by the electrostatic transducer and the capacitor C2 is driven as the load capacitor CL.
Referring to the circuit shown in FIG. 15, in a fourth-order LC filter that is configured to include L1, C1, L2, C2, and RL and is terminated at one end of the circuit, examples of an output voltage (terminal voltage of C2), power supplied to a load capacitor C2 (CL), and a loss being consumed in a load resistor RL (used as a damping resistor) assuming that C2 is used as the load capacitor (for example, CL=5 nF) are shown in FIG. 16.
FIG. 16 is a view illustrating an example of a loss occurring in the load resistor RL when directly driving a load capacitor with a class-D power amplifier and an LC filter. As shown in FIG. 16, a flat output characteristic (output voltage) can be obtained with the load resistor RL having a proper resistance value. Instead, a loss much larger than the power (apparent power) supplied to the load capacitor occurs in the damping load resistor RL (refer to load resistor loss data in FIG. 16). As a result, an unnecessary loss occurs, and thus the circuit efficiency ranging from an amplifier to a load is lowered. In other words, in the case of driving a typical loudspeaker in the class-D power amplifier, a load resistor itself is a speaker and the unnecessary loss does not occur in portions other than a load to be driven, and as a result, it is possible to improve the circuit efficiency ranging from an amplifier to a load.
As described above, when directly driving the load capacitor C in the configuration of the class-D output stage and the LC filter which is a configuration of a typical class-D power amplifier for audio, an unnecessary loss occurs in the damping load resistor RL in the case of intending to obtain a flat output characteristic. As a result, a serious problem occurs in that the efficiency of the entire driving circuit is noticeably lowered. This is not preferable because the characteristic of the class-D power amplifier having high efficiency cannot be applied to a system.
FIG. 17 is a view illustrating a frequency characteristic of an output voltage in the case when there is no load resistor RL shown in FIG. 15. If the load resistor RL is removed or is changed to a resistor having a high resistance value in order to reduce the loss in the damping load resistor RL, a resonance characteristic of the LC filter becomes noticeable and the frequency characteristic of the output voltage largely varies as shown in FIG. 17. In an example shown in FIG. 17, since a characteristic around a driving frequency band of an ultrasonic speaker varies largely, it is not possible to stably drive the ultrasonic speaker with the characteristic described above.
As described above, in the case when the class-D power amplifier is used in a driving circuit of the electrostatic transducer, if a damping resistor is removed or is changed to a resistor having a high resistance value in order to reduce the loss in the damping resistor, the resonance characteristic of the LC filter becomes noticeable and the frequency characteristic of the output voltage largely varies as shown in FIG. 17. In the example shown in FIG. 17, since the characteristic around the driving frequency band of the ultrasonic speaker varies largely, a problem has occurred in that it is not possible to stably drive the ultrasonic speaker with the characteristic described above.